This study evaluated which CT findings could be used to predict the negative results of a sputum smear in patients with active pulmonary tuberculosis and multiple cavities.

Thirty-eight patients with active pulmonary tuberculosis and multiple cavities on CT were classified into 2 groups: smear-positive (n = 30) and -negative (n = 8). The CT findings were reviewed retrospectively. The maximum internal diameter of the largest cavity, the number of the cavities and lobes with cavities, and the characteristics of the associated findings such as consolidation, ground glass opacity, micronodules and nodule were accessed. The number of cavities above 20 mm in the maximum internal diameter and a necrotizing pneumonia-like pattern were also evaluated.

The maximum internal diameter and number of cavities was 32.23 ± 17.66 mm and 15.50 ± 11.12 mm (p = 0.0042), and 5.53 ± 3.17 and 2.43 ± 1.13 (p = 0.0002) in the smear-positive and -negative group, respectively. Three or more cavities were observed at 76.7% and 12.5% in the smear-positive and -negative group, respectively (p < 0.005). There were 2.00 ± 1.34 and 0.25 ± 0.46 (p = 0.001), and 1.5 ± 1.50 and 0.38 ± 0.52 (p = 0.0016) lobes with consolidation and ground glass opacity in the smear-positive and -negative group, respectively. A necrotizing pneumonia-like pattern was observed in 43.3% of the smear-positive group only. The other findings were similar in both two groups. The sensitivity, specificity, positive and negative predictive value for the presence of at least a finding of consolidation, more than 3 cavities or the largest cavity > 20 mm in the maximum internal diameter were 100%, 62.5%, 90.9%, and 100%, respectively.

Two cavities 20 mm or less in the maximum internal diameter without consolidation on CT might be associated with a negative result of the sputum smear in patients with active pulmonary tuberculosis and multiple cavities.